If you're still new to the world of testing in your applications, you should give this new guide from Gundars Meness a read. He's done a great job of providing "the big picture" of testing - why to do it and what it is.

This is not a crash course of what characters one needs to type in his code editor to produce unit tests. This is fuel the brain requires before attempting such actions.

The subject of Unit Testing is not as simple as one might think. Many of us, developers, go into unit testing based on pressure from clients, peers, colleagues, our heroes and so on. We quickly learn the value of it, and, once the tech setup is done, there is a tendency to forget the big picture, if it was ever learnt. This article will provide a short insight into what is and isn’t unit testing in PHP and in general, and unit test place in the quality assurance realm.

He then breaks up the article into a few different sections, each with a paragraph or two, covering different testing related topics:

What is testing?

What really is testing?

What is Unit Testing?

How to write a Unit test?

There's some great suggestions in here, for both those new to testing and those still trying to figure out some good practices.